Trace (<5%) impurity seeding of boron and carbon by laser blow-off is used to study low-Z parallel and radial impurity transport in the TJ-II stellarator. Toroidal transport is observed to be quite rapid τϕ ∼ 0.1 ms, roughly consistent with expectations of collisionless (kinetic) parallel ion transport. Radial impurity de-confinement of injected impurities to the plasma wall is measured to be much slower, τr ∼ 10 ms. The data suggest a slightly higher τr for carbon than for boron, but no clear charge state dependence is observed in contrast with expectations for neoclassical transport in the low collisionality regime.
Overview of recent TJ-II stellarator results